Vegetable oil triglycerides have been available for use in food products and cooking. Many such vegetable oils contain natural antioxidants such as phospholipids and sterols that prevent oxidation during storage. Triglycerides are considered the esterification product of glycerol with 3 molecules of carboxylic acids. The amount of unsaturation in the carboxylic affects the susceptibility of the triglyceride to oxidation. Oxidation can include reactions that link two or more triglycerides together through reactions of atoms near the unsaturation. These reactions can form higher molecular weight material which can become insoluble and discolored e.g. sludge. Oxidation can also result in cleavage of the ester linkage or other internal cleavage of the triglycerides. The fragments of the triglyceride from the cleavage, being lower in molecular weight, are more volatile. Carboxylic acid groups generated from the triglyceride make the lubricant acidic. Aldehyde groups can also be generated. Carboxylic acid groups have attraction for oxidized metals and can solubilize them in oil promoting metal removal from some surfaces.
Due to oxidation problems with triglycerides most commercial lubricants are formulated from petroleum distillates which have lower amounts of unsaturation making them resistant to oxidation. Petroleum distillates require additives to reduce wear, reduce oxidation, lower the pour point and modify the viscosity index (to adjust either the high or low temperature viscosity) etc. The petroleum distillates are resistant to biodegradation and the additives used to adjust their characteristics (often containing metals and reactive compounds) further detract from the biodegradability of the used lubricant.
Synthetic ester lubricants having little or no unsaturation in the carbon to carbon bonds are used in premium quality motor oils due to their desirable properties. However the acids and alcohols used to make synthetic ester usually are derived from petroleum distillates and are thus not from a renewable source. They are also more costly and less biodegradable than natural triglycerides.
U.S. Pat. No. 4,867,890 discloses the use of soluble copper compounds to prevent oxidation in mineral oil lubricants with an ashless dispersant and zinc dihydrocarbyldithiophosphate. Therein effective amounts of copper were described as from about 5 to about 500 parts per million.